The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
Pathogenic organisms are, by definition, capable of causing disease in an infected host. For clinical use of such organisms, attenuated vaccine strains are often created which exhibit reduced or eliminated virulence, but which still retain sufficient viability to stimulate a desired immune response against the pathogen or heterologous antigen(s) of interest. Attenuated vector platforms have been demonstrated to elicit protective responses specific for encoded heterologous antigens in a number of experimental models, including infectious and malignant diseases.
Although most attenuated vaccine vectors are viral, bacterial vaccine vector platforms have been developed for both prophylactic and therapeutic applications. Attenuated strains of many otherwise pathogenic bacteria are now available and the ease of manipulation for generating recombinant strains provides a means for using bacteria as efficacious delivery vehicles for a number of foreign proteins such as antigens associated with infectious diseases and cancer. Live attenuated bacterial vaccine strains have been developed from, inter alia, Listeria, Escherichia, Salmonella, Shigella, Lactobacillus, and Yersinia species.
Regulating the level of heterologous antigen expression can have a significant impact on the immunogenicity of the vaccine. In bacterial vaccine vectors, the heterologous gene encoding the vaccine antigen can be either integrated into the bacterial chromosome or expressed from a plasmid. Chromosomal integration allows maximum genetic stability. However, chromosomal integration usually results in a single copy of heterologous antigen per bacterium, and it is a challenge to ensure that sufficient antigen is expressed to confer protective immunity. In plasmid-based expression, spontaneous loss of plasmid can result in plasmid-less bacteria rapidly outgrowing plasmid-bearing bacteria and becoming the dominant population in tissues.
There remains a need in the art to provide systems and methods to provide bacterial vaccine strains with advantageous expression levels of heterologous antigens for use in the treatment or prevention of diseases.